I have a question about open back subwoofersI have noticed some multiple driver systems have drivers all facing one way and others(HAL)has the drivers alternating direction. What is the advantage of this?

From what I understand, the cones to not respond exactly the same in either direction due to construction (suspension, etc.). Using two drivers, mounted inverted to each other with phase reversed to one of the drivers (as you know, LOL) allows the drivers to cancel each other's inaccuracies.

I'm probably wrong :(

Not sure, but I've got a feeling that other issues may be more important in the real world. Especially as the frequencies come up. I sure wouldn't use that construction in any speaker expected to have output over ~120Hz with less than a 4th order cutoff. You can easily hear frequencies out of the LF drivers well above that when isolating/testing the drivers. When I had my Tympanis, the quality of the bass amp had a big impact on midrange clarity. That was a 3rd order cutoff @ ~240Hz. When designing your crossover (or adjusting your electronic one) that output becomes significant.

From what I understand, the cones to not respond exactly the same in either direction due to construction (suspension, etc.). Using two drivers, mounted inverted to each other with phase reversed to one of the drivers (as you know, LOL) allows the drivers to cancel each other's inaccuracies.

I have a question about open back subwoofersI have noticed some multiple driver systems have drivers all facing one way and others(HAL)has the drivers alternating direction. What is the advantage of this?

All sound waves created by the rear face of the speaker cones are essentially distortions in real world, unless the speaker is located in a place where reflections from the rear walls is absent. To eliminate the rear reflections, speakers manufacturers mane great efforts. Tom’s eggs are a great example. The goal of the eggs and egg base is to minimize the rear reflections effect on the cones.

Not quite correct on the eggs. The problem with many enclosures is diffraction from the edges from the front pressure wave and internal standing waves from the rear wave. The egg shape of the upper section and assymmetrical truncated pyramid shape of the bases perform two functions.

The egg shape minimizes diffraction from the front of the mid and tweeter. The continuously varying shape of the inside minimizes both diffraction and standing waves on the inside.

The truncated pyramid minimizes standing waves on the inside and the rounded top of the pyramids blending into the base of the eggs minimizes diffraction of the pressure waves from the mid and tweeter as they pass down toward the base. The varying distances from the bass drivers to the edges of the pyramids also minimizes diffraction build up of the low frequencies at the edges of the base. Because of the spread of the distances to the edges of the bases and the wavelengths involved from the bass drivers. room contributions are a far more significant percentage of the problem at those low frequencies.

Thanks TomHow does it cancel 2nd order harmonics and nothing elseI understand that you have to wire opposing drivers out of phase

Actually I should have been clearer when I said it reduced the distortion. Since the second order distortion can be different in the two directions, when they are mounted facing the same direction the distortion from both drivers stays the same in one direction. Since distortion measurements do not take into account direction the distortion is the same. When they are mounted in opposite directions you get the higher distortion on any half cycle only from one driver.

Just for grins I'll put some numbers to it. If each driver creates 10% distortion in one direction and 0% in the other direction (it actually won't happen that way) the total distortion in one direction will be 10% and 0% in the other. the reason the distortion numbers as a percent do not add is that each driver is not only adding more distortion, but more fundamental sound as well. therefore the distortion percentage remains constant. If the two drivers are reversed, the distortion will now be 5% in each direction, 10% from one and 0% for the other in each direction. Remember it is a percent not absolute pressure. Since both the "good" and "bad" driver are creating the same amount of the fundamental but only one is now creating the distortion, the fundamental doubles but the distortion remains the same, therefore the % halves.

I have only heard this effect mentioned as far as second order but perhaps it works for other orders as well but that may depend on the driver construction and geometry to a greater degree for higher orders. I have no clue. However, although this may reduce some distortion, the 800 pound gorilla in the room is always insufficient linear displacement from the drivers. Once you exceed XMax, the distortion rises dramatically which is far more noticeable than whether or not the drivers are mounted the same way or not. The opposite direction mounting is just the frosting on the cake. If the drivers are mounted in such a way that the view of the back side of the driver is hidden it will not hurt to mount them in opposite directions. I did not do that with the eggs because with low distortion drivers to start with the acoustical gain was not enough to have to look at the back side of one of the drivers. It would have totally destroyed the visual effect of these speakers for very little advantage.

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